Reliability is more important than availability in the long run. Clients who know when you are not available can make informed choices regarding alternative sources of support.
I once worked with a client who rang her GPs so frequently and insistently that they established a rota for taking her calls. She bombarded every new therapist with telephone calls. I told her she could call me between 1030 and 1130 on Monday or Thursday and that if I was on another call, I’d call her as soon as I finished. She rang me twice the first week and two more times in the next six months. She also called her GPs and CPN less frequently.
Many therapists feel a duty to respond to client’s crises. Who better to address a difficult situation: the therapist who has listened carefully to the client’s life-story or the harassed junior medic who has never met them before? Shouldn’t you always be available for your client?